After years of engaging in far-flung operations from Afghanistan to Africa, NATO is returning to its core mission. Next month’s NATO summit in Warsaw will complete the transatlantic alliance’s transformation back to what it was built for: deterring Moscow.

The reason for NATO’s return to its old mission is Russia’s return to its old ways. Poland’s defense minister calls Russia “an aggressive state.” Likewise, U.S. Air Force Gen. Phillip Breedlove, who recently ended his tour as NATO commander, worries about “a resurgent and aggressive Russia.” For too long, according to Breedlove, NATO has “hugged the bear”—a reference to Western efforts to reassure Russia, carve out a special Russian place within NATO headquarters, overlook Moscow’s mischief, and downgrade the quantity and quality of military assets in Europe. What NATO has gotten in return, Breedlove concludes, is a Russia that is “blatantly attempting to change the rules and principles that have been the foundation of European security for decades.”

Consider the record: Vladimir Putin’s Russia has lopped off part of Georgia, annexed Crimea and occupied eastern Ukraine, waged cyberwar against the Baltics, threatened Poland with nuclear attack, raised the prospect of using nuclear weapons to de-escalate a conflict, massed troops on the borders of NATO’s newest members, flouted arms treaties, and revived the dangerous Cold War-era practice of conducting mock bombing runs, buzzing Allied warships and testing Allied air defenses. There were 160 Russian incursions into Baltic airspace in 2015.

Another 2015 data point: Putin unveiled a new military doctrine focused on confronting NATO and pledging the use of Russia’s armed forces “to ensure the protection of its citizens outside the Russian Federation.” Given that there are five million Russians in Ukraine and a million in the Baltics—and that Putin has reserved for himself the right to determine when, where and whether they need to be protected—this is a recipe for something much more complicated than a new cold war. As if to underscore his intentions, Putin recently reactivated the 1st Guards Tank Army, a large armored force based in western Russia equipped with some 500 T-72 and T-80 main battle tanks.

Between 2004 and 2013, Putin—sometimes as prime minister, sometimes as president—increased military spending 108 percent. Russia’s 2015 military outlays were 26-percent larger than in 2014.

In short, even as NATO tried to build bridges to Moscow, even as NATO avoided building bases in Eastern Europe, even as NATO states slashed defense spending, even as NATO offered partnership to Russia and membership to Eastern Europe, Putin was longing for the bad old days. As the Brookings Institution’s Robert Kagan concludes, “It is the entire post-Cold War settlement of the 1990s that Russia resents and wants to revise.”

Perhaps with that goal in mind, Putin boasts, “If I wanted, Russian troops could not only be in Kiev in two days, but in Riga, Vilnius, Tallinn, Warsaw or Bucharest.” All of these cities were under Moscow’s heel during the Cold War.

Putin rationalizes his belligerence by arguing that NATO started it. For instance, Russia’s recently-released national security strategy blames tensions in Europe on “expansion of the alliance.” This echoes Putin’s assertion that NATO violated agreements at the end of the Cold War not to expand.

The only problem with Putin’s narrative is that it doesn’t correspond with history or reality. As Steven Pifer details, Mikhail Gorbachev “made clear there was no promise regarding broader enlargement.” Gorbachev himselfconcedes, “The topic of NATO expansion was not discussed at all.”

So, the alliance didn’t double-cross its way to the Russian border. In fact, NATO grew through a transparent and deliberate process that allowed East European states to pursue or decline membership on their own volition—a process that encouraged the sort of political, institutional and economic reforms that actually diminished tensions with post-Soviet, pre-Putin Russia. But under Putin, “Russia has bad relations with all the democratic countries on its borders,” as Estonian President Toomas Hendrik Ilves observes. “That should make one think.”

Permanence

Given Putin’s record and rhetoric, it’s no surprise that political leaders from NATO’s easternmost members—Lithuania, Latvia, Estonia, Slovakia, Bulgaria, Hungary, Romania, Poland and the Czech Republic—want “a robust, credible and sustainable Allied military presence in our region.” To buttress their request, they cite “the aggressive Russian actions in Ukraine, including the illegal and illegitimate annexation of Crimea, the support to separatists in Ukraine, as well as Russia’s military activities in our neighborhood.”

Their worries are well-founded.

Within range of Putin’s unmarked armies and clever brand of anonymous warfare, Eastern Europe’s leaders understand that if Putin follows his Ukraine playbook and covertly violates the sovereignty of the Baltics, he will force NATO to blink or fire back. Neither alternative leads to a happy outcome. The former means NATO is neutralized and neutered. (Putin’s goal, as Breedlove explains, is to “fracture” NATO.) The latter means war.

The best way to prevent those dire scenarios is to answer Eastern Europe’s SOS and base permanent NATO assets where they are most needed: on the territory of NATO’s most-at-risk members. That’s what the alliance did during the Cold War, and it kept the peace—as it will today.

But don’t take my word for it. Take it from those who specialize in deterrence: Retired U.S. Army Gen. Robert Scales advocates deployment of “a heavy-armored brigade in each Baltic state and two in neighboring Poland.” British Gen. Richard Shirreff, former deputy commander of NATO, says the best way to prevent war in Europe is “to maintain troops permanently in the Baltic states.” NATO’s new military commander, U.S. Army Gen. Curtis Scaparrotti, wants “a permanently stationed armored brigade in Europe.”

Good News, Bad News

The good news is that Putin now faces a NATO alliance that seems reunified and renewed in purpose.

Sixteen NATO members increased defense spending in 2015. European defense spending is up 8.3 percent in 2016. Germany, for the first time in 25 years, will expand its military endstrength by some 14,300 personnel. Washington has quadrupled U.S. military spending earmarked for Europe—from $789 million to $3.4 billion. A U.S. defense official says NATO is “moving from assurance to deterrence.”

Toward that end, after years of waning commitment, the U.S. Army is increasing its deterrent strength in Europe by permanently basing threefully-manned brigades on the continent. NATO is hammering out plans to deploy four battalions—one each in Poland, Latvia, Estonia and Lithuania—to deter Putin from the sort of ambiguous, anonymous warfare he has waged in Ukraine. And importantly, Scaparrotti comes to NATO not from the counterinsurgency campaigns of the Middle East, or the nation-building mission in Afghanistan, or the shadow wars against ISIS and al Qaeda, but from the 38th Parallel, where U.S. and South Korean troops serve as a 24/7 deterrent against North Korean invasion.

The bad news is that most NATO members have been hacking away at NATO’s deterrent strength, which explains NATO’s urgent call that each member invest at least 2 percent of GDP in defense. Only five of NATO’s 28 members—the United States, Britain, Greece, Poland and Estonia—meet that standard today. Britain is using accounting tricks to remain above the 2-percent threshold. U.S. defense spending has tumbled from 4.7 percent of GDP in 2010 to 3.1 percent today, headed for 2.7 percent by 2019.

Years of underfunding have led to “alarming deficiencies in the state of NATO preparedness,” according to the British government.

For example, post-recession austerity measures have reduced the Royal Navy from 89 ships to 65. Britain had two aircraft carriers in 2008 but none today. Britain’s combat-aircraft fleet has shrunk from 189 warplanes to 149; the Joint Helicopter Command had 257 aircraft in 2008 but just 164 today.

Only 42 of Germany’s top-of-the-line 109 Eurofighters are in flying condition. At the height of the Cold War, West Germany had 2,125 Leopard II tanks. Today, the German army has just 225.

The French military eliminated 8,000 personnel in 2014, mothballed 19 warships between 2009 and 2012, and slashed the size of its air fleet by 30 percent between 2008 and 2013, as AEI details.

The U.S. Army has around 26,000 troops in Europe today, down from 40,000 in 2012, down from 300,000 at the height of the Cold War. As a consequence, Army brass are trying “to make 30,000 look and feel like 300,000,” according to Gen. Ben Hodges, commander of U.S. Army-Europe.

Moreover, Washington’s response to Russian aggression—and to Allied anxieties—reflects the problematic “lead from behind” approach that characterized most of President Obama’s foreign policy. Thus, rather than a robust commitment of a brigade or more in the Baltics and Poland, the Obama administration is offering a battalion to NATO’s tripwire force in Eastern Europe—and apparently reneged on earlier pledges of two U.S. battalions.

Even so, although a few brigades would be better than a few battalions, the Baltic-battalions option is better than what NATO offered its easternmost members before Crimea—which was nothing.

Signals

Despite Washington’s halfhearted reaction, what NATO’s easternmost members are requesting is feasible, compatible with NATO’s core mission and militarily credible.

First, NATO has about 3.3 million men under arms and accounts for 60 percent of world military spending. In other words, the alliance can do this—but only with a renewed commitment to its enduring mission of deterrence and a renewed commitment from each member.

Each NATO member should lift its defense budget to the 2-percent-of-GDP standard by a date certain; each member should invest in such a way that serves the needs of the alliance; and Washington should lead from the front by reversing sequestration’s devastating cuts.

Second, basing a deterrent force in Eastern Europe is in line with NATO’s core mission.

Since its founding, the NATO deterrent has been an insurance policy against worst-case scenarios. For the United States, NATO diminishes the likelihood of another European conflict triggering another great-power war. For NATO’s other members, NATO is a security guarantee backed by the United States. Without that guarantee, there is no security, as history has a way of reminding those on the outside looking in, from Cold War Hungary to post-Cold War Ukraine.

Indeed, the reason Poland wants U.S. troops on Polish soil is the same reason U.S. troops were based in West Berlin during the Cold War and have been based along the 38th Parallel since 1953: American troops send an unmistakable message that crossing this line means you are going to war against the United States—no ambiguity, no question marks, no doubts about the consequences. This is the essence of deterrence, and it works.

Third, permanent bases in the Balts and Poland would signal Moscow that NATO is serious about defending Eastern Europe. The goal is not to start a war but quite the opposite: to prevent what Churchill called “temptations to a trial of strength.”

Russia’s military high command knows Baltic-based battalions (or brigades) could not invade Russia. However, having American personnel on the ground changes the calculus in Moscow and will make it clear to Putin that NATO’s security guarantee is as valid for NATO’s youngest members as it is for NATO’s oldest members.